Re: [PATCH v2] ext4: fix hole punch failure when depth is greaterthan 0

[Lukáš Czerner]

On Thu, 5 Jul 2012, Ashish Sangwan wrote:

> Date: Thu, 5 Jul 2012 15:22:04 +0530
> From: Ashish Sangwan <ashishsangwan2@xxxxxxxxx>
> To: LukÃÅ Czerner <lczerner@xxxxxxxxxx>
> Cc: sandeen@xxxxxxxxxx, Ted Tso <tytso@xxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx,
>     linux-ext4@xxxxxxxxxxxxxxx, Namjae Jeon <linkinjeon@xxxxxxxxx>
> Subject: Re: [PATCH v2] ext4: fix hole punch failure when depth is greater
>     than 0
> 
> On Wed, Jul 4, 2012 at 11:03 PM, LukÃÅ Czerner <lczerner@xxxxxxxxxx> wrote:
> > So I've finally has some time to look at the patch and reproduce the
> > problem. Thanks for noticing the problem, the patch seems good,
> > though I have one question. Is the p_block setting really necessary
> > ? I do not think so, but I might be missing something. Here is
> > updated patch I've tested, bellow.
> AFAICS, p_block setting is necessary. As mentioned in the patch description,
> whether to continue removing extents or not is decided by the return value
> of function ext4_ext_more_to_rm() which checks 2 conditions:
> a) if there are no more indexes to process.
> b) if the number of entries are decreased in the header of "depth -1".
> The p_block setting is important for condition b).
> 
> In function ext4_ext_more_to_rm, there is this second check:
> if (le16_to_cpu(path->p_hdr->eh_entries) == path->p_block)
>     return 0;
> If the value of p_block would not be correct, the above mentioned
> condition could become
> true while there are still blocks left to be removed.

Ok, the code is not very clear, but now I can see it. p_block is
actually misused here to store the number of indexes in the current
node while diving into the tree. Then on the way up, we are checking
that to see if the eh_entries changed or not (which is indicating
that something has been freed deeper in the tree).

That said, it makes sense to set it before the loop itself because
we are actually skipping the path construction while diving into
the tree since patch is already initialized and we're starting
walking back from 'depth' up in this case. So the patch seems fine.
Thanks for catching it and fixing it.

You can add

Reviewed-by: Lukas Czerner <lczerner@xxxxxxxxxx>


> >
> > Note: there are some indent problems in your patch, like for example
> > this:
> >
> > +                       path[k].p_block =
> > +                       le16_to_cpu(path[k].p_hdr->eh_entries)+1;
> >
> >
> Before submitting the patch, I run checkpatch.pl with --strict option.
> It did'nt show any error or warning. Should I re-submit
> the patch with an extra tab before the second line? The call is yours.

checkpatch.pl does not catch everything. Just look at how wrapping
of long lines is done in the code, there are plenty of examples.

> 
> > Anyway, what do you think about the modification ?
> >
> Also there is 1 modification missing from your patch.
>     ext4_ext_drop_refs(path);
>         kfree(path);
> +       path = NULL;
>         if (err == -EAGAIN)
>                 goto again;
> If path is not set to NULL, it will crash in xfstest #13. Ted has
> already reported it.

Right, I've probably used the old patch as an example.

Thanks!
-Lukas

> 
> Thanks,
> Ashish
> 
> > Thanks!
> > -Lukas
> >
> >
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index b3300eb..94bc1bd 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -2570,7 +2570,7 @@ static int ext4_ext_remove_space(struct inode *inode, ext4_lblk_t start,
> >  {
> >         struct super_block *sb = inode->i_sb;
> >         int depth = ext_depth(inode);
> > -       struct ext4_ext_path *path;
> > +       struct ext4_ext_path *path = NULL;
> >         ext4_fsblk_t partial_cluster = 0;
> >         handle_t *handle;
> >         int i, err;
> > @@ -2606,8 +2606,12 @@ again:
> >                 }
> >                 depth = ext_depth(inode);
> >                 ex = path[depth].p_ext;
> > -               if (!ex)
> > +               if (!ex) {
> > +                       ext4_ext_drop_refs(path);
> > +                       kfree(path);
> > +                       path = NULL;
> >                         goto cont;
> > +               }
> >
> >                 ee_block = le32_to_cpu(ex->ee_block);
> >
> > @@ -2637,8 +2641,6 @@ again:
> >                         if (err < 0)
> >                                 goto out;
> >                 }
> > -               ext4_ext_drop_refs(path);
> > -               kfree(path);
> >         }
> >  cont:
> >
> > @@ -2646,20 +2648,26 @@ cont:
> >          * We start scanning from right side, freeing all the blocks
> >          * after i_size and walking into the tree depth-wise.
> >          */
> > -       depth = ext_depth(inode);
> > -       path = kzalloc(sizeof(struct ext4_ext_path) * (depth + 1), GFP_NOFS);
> > -       if (path == NULL) {
> > -               ext4_journal_stop(handle);
> > -               return -ENOMEM;
> > -       }
> > -       path[0].p_depth = depth;
> > -       path[0].p_hdr = ext_inode_hdr(inode);
> > +       if (path)
> > +               i = depth;
> > +       else {
> > +               depth = ext_depth(inode);
> > +               path = kzalloc(sizeof(struct ext4_ext_path) * (depth + 1),
> > +                              GFP_NOFS);
> > +               if (path == NULL) {
> > +                       ext4_journal_stop(handle);
> > +                       return -ENOMEM;
> > +               }
> > +               path[0].p_depth = depth;
> > +               path[0].p_hdr = ext_inode_hdr(inode);
> >
> > -       if (ext4_ext_check(inode, path[0].p_hdr, depth)) {
> > -               err = -EIO;
> > -               goto out;
> > +               if (ext4_ext_check(inode, path[0].p_hdr, depth)) {
> > +                       err = -EIO;
> > +                       goto out;
> > +               }
> > +               i = 0;
> >         }
> > -       i = err = 0;
> > +       err = 0;
> >
> >         while (i >= 0 && err == 0) {
> >                 if (i == depth) {
> >
> > On Mon, 2 Jul 2012, Ashish Sangwan wrote:
> >
>